Wagner has played for the German youth national teams, winning a gold medal at the 2018 Albert Schweitzer Tournament.

[3] On July 6, 2019, Wagner committed to play college basketball for Michigan under head coach Juwan Howard.

[6] He made his season debut for the Wolverines on November 27, 2019, and finished the game with six points, three rebounds, and one block in 23 minutes in an 83–76 victory over Iowa State in the quarterfinals of the Battle 4 Atlantis.

[7][8] On December 6, Wagner posted a career-high 18 points in a 103–91 victory over Iowa in its Big Ten Conference season opener, as Michigan had six double-digit scorers, including all five starters.

[19] In Game 2 of the BBL Finals against Bayern Munich, Wagner scored a team-high 14 points, making all six of his shots.

[23] Wagner posted his first NBA double-double with a career-high 11 rebounds and 14 points as well as 6 assists on December 18 against the Brooklyn Nets.

[26] Wagner's second NBA double-double occurred on January 12, 2022, against the Washington Wizards when he upped his career-high assist plateau from 6 to 10 and added 14 points.

[29] On November 5, 2022, Wagner scored a season-high 31 points and recorded six assists in a 126–123 overtime loss to the Sacramento Kings.

[32] On April 27, 2024, Wagner recorded 34 points on 13-of-17 shooting, along with 13 rebounds to lift the Magic to a 112–89 victory in Game 4 as Orlando tied their First Round series with the Cleveland Cavaliers 2–2.

[35] On November 21, Wagner had a season-high 37 points, along with six rebounds, 11 assists, four steals and scored a game-winning three-pointer in a 119–118 victory against the Los Angeles Lakers.

[41] In the 2019 FIBA U18 European Championship in Volos, Greece, he averaged 13 points and 4.8 rebounds per game, leading his team to 11th place.